The sudden popularity of the Internet as a communication tool has led to an intense push for higher data transmission rates over the Public Switched Telephone Network (PSTN). As a result, the demand for increased data transmission rates over analog twisted pair wiring is at an all time high. The most recent widespread standard is “56K” analog modem technology developed by U.S. Robotics and Rockwell/Lucent. While these technologies generally have not generated true 56 kbps performance under typical subscriber line conditions, they do provide a boost in performance from the previous standard of bidirectional 33.6 kbps.
Theoretically, a connection of 64 kbps should be attainable between the subscriber and the Internet Service Provider (ISP) via a standard Plain Old Telephone Service (POTS) connection. This is because 64 kbps is the rate at which data is transferred from the Central Office (CO) linecard to the ISP or other remote terminal. Several factors prevent this from happening including imperfect line conditions and varying local loop lengths common to POTS analog networks. The primary reason, however, for this less than the theoretical transmission rate is that the PSTN was designed to carry voiceband frequencies in the range of 300 Hz–3.4 kHz.
With the advent of digital voice systems, the decision was made to use a “companded” (compressed/expanded) data to reduce the number of bits per digital sample from a nominal 13-bits to 8-bits. Companding schemes use higher resolution at low signal amplitudes and lower resolution at high amplitudes. Companded signals are suitable for voice frequencies but not for analog modems since they limit their apparent bandwidth to a ceiling of 33.6 kbps. In practice, most analog modems are only able to achieve rates of 46–48 kbps downstream due to less-than-perfect analog line conditions.
The Analog-to-Digital (“A/D”) portion of the linecard coder/decoder (“codec”) is where the analog signal is converted to its 8-bit companded representation. Hence, the linecard codec acts as a bottleneck in the entire data communications chain. One way of avoiding this bottleneck is by removing the A/D conversion in the downstream direction. This is accomplished by requiring a digital connection between the provider and its CO and increasing the data throughput of the modem signals to capitalize on the extra capacity. This is the basis of 56k technologies.
Moreover, while the use of 56K standards results in downstream data throughput of 56 kbps under ideal local loop conditions, the upstream direction must still contend with an A/D conversion into 8-bit companded data and is still limited to 33.6 kbps. Imperfect conditions in the analog local loop further degradate the signal resulting in less than the 56/33.6 kbps maximums.
Additionally, while 56K standards offer improvements over the older V.34+ standard, bandwidth is still needed to keep pace with upcoming technologies such as video conferencing, remote server access, and other high rate transmission protocols. If higher data throughput is to be achieved, the limitations in the CO need to be overcome. Overhauling the PSTN by replacing the 8-bit companded data scheme could solve the problem, but this is not a feasible solution since the cost of such as effort would be enormous.